The present invention relates to a manually guided implement having an internal combustion engine.
Implements such as chainsaws, brush cutters or the like are provided with a diaphragm carburetor for forming an air/fuel mixture for the internal combustion engine that drives the implement. To set a desired mixture ratio of the air/fuel mixture, the diaphragm carburetor is provided with a mixture setting device that includes a main nozzle and an idling nozzle. The fuel throughput through the nozzles, and hence the mixture ratio, is a function of pressure in the venturi section of the intake channel and can, within prescribed limits, be adapted via adjustment screws. The nozzles are supplied from a fuel-filled control chamber that is supplied with fuel via a feed valve that is controlled by a control diaphragm. In order to keep the preset mixture ratio constant as a function of a variable reference pressure, there is provided on the dry side of the control diaphragm a compensation chamber that is supplied with the reference pressure. The reference pressure can, for example, be the ambient pressure, which is dependent upon the weather or site of use, or can be the intake pressure on the clean air side of an intake air filter, with such intake pressure being dependent upon how dirty the air filter is.
These implements, at a high output of the internal combustion engine and a low emission of pollutants, are operated with a lean setting of the mixture ratio. However, at high speeds a lean setting can lead to increased thermal stress.
Internal combustion engines, especially two-stroke engines, that are set lean additionally tend to reach high speeds at full throttle without the application of an external load. If during operation the external load, for example, suddenly drops, and if the operator reduces the fuel feed only with a time delay, the internal combustion engine can reach damaging excess speeds.
In practice two-stroke engines are therefore set richer in the full throttle range, as a result of which not only the thermal stress is reduced, but the maximum speed is also lowered. The drawback of a richer mixture setting is the loss of power that inherently results, and also the increased fuel consumption.
To limit the maximum speed of a two-stroke engine, mechanical regulators, which are controlled by centrifugal force, or electronic regulators are known, according to which when a prescribed speed is exceeded, this speed is reduced by suppressing the ignition. The drawback of such regulators is that they become effective only after the prescribed speed has been exceeded. After the regulation process has started, the speed is then again reduced to a value at which the regulator is no longer active. The speed again increases until the regulation process is again initiated. An internal combustion engine, the speed of which is limited in this fashion, tends to oscillate back and forth between an upper and a lower threshold speed. A precisely defined speed limitation can therefore be achieved only with difficulty. In addition, interruption of the ignition allows unburned fuel to be discharged. If a catalytic converter is provided in the exhaust system, the fuel is after burned and an increased quantity of heat is released, which thermally stresses the catalytic converter.
It is therefore an object of the present invention to improve an implement of the aforementioned general type having an internal combustion engine in such a way that with straightforward means the fuel consumption as well as the thermal and mechanical stressing of the internal combustion engine are reduced.